This invention relates to liquid crystal materials and, more particularly, to Fresnel lenses that include such materials as constituents.
In a variety of optical information processing, interconnection and communication applications, it is advantageous to be able to focus light beams that are propagated between components. Thus, for example, in a system in which an array of lasers is provided, a corresponding array of lenses may be required to focus the individual beams emitted by the lasers, thereby to avoid deleterious beam dispersion effects. Fresnel lenses aligned with the outputs of such lasers constitute a convenient way of focusing their respective beams. An advantageous integrated assembly comprising a laser array with associated Fresnel microlenses formed on a single chip is described in a co-pending commonly assigned U.S. patent application of K. Rastani (Case 1), Ser. No. 612,924, filed Nov. 13, 1990.
Fresnel microlenses of the type described in the aforecited Rastani application are characterized by predetermined optical properties. The lenses in such as assembly are each designed to have an optimal diffraction efficiency at a particular wavelength. (In a batch-fabricated assembly, all the microlenses would typically be designed for the same wavelength). Once fabricated, such a lens exhibits fixed optical properties which would be less than ideal at other than the particular wavelength for which the lens was designed.
A Fresnel lens based on the use of liquid crystal materials has been suggested, as described by G. Williams et al in an article entitled "Electrically Controllable Liquid Crystal Fresnel Lens", SPIE, number 1168, pages 352-357, 1989. In the arrangement described by Williams et al, two identical Fresnel lenses are aligned in cascade such that the optic axis of one lens is orthogonal to that of the other. In that way, polarization-independent operation at a variable wavelength selected by varying an applied electrical bias voltage is reportedly achieved.
But unless the two constituent lenses of the Williams et al arrangement are exactly aligned with respect to each other, the focus achieved by the arrangement for an optical beam of one polarization will be spatially displaced along a given axis with respect to the focus achieved for an orthogonally polarized beam. Moreover, even if the constituent lenses are exactly aligned (which is particularly difficult to realize in the case of microlenses), the cascaded lenses of Williams et al will inherently form respective spaced-apart foci along an axis that is perpendicular to the given one. In practice, such polarization-dependent spatial variations in focus are undesirable.
Another disadvantage of the Williams et al design is that it includes patterned electrodes in the form of zones. Such an electrode pattern causes fringe electric fields, which are undesirable.
Accordingly, efforts have been directed at trying to develop electrically controlled polarization-independent Fresnel lenses that are relatively easy to fabricate, that do not exhibit spatial variation in focus for orthogonal polarizations and that do not exhibit fringe field problems. It was recognized that these efforts, if successful, would significantly contribute to the realization of high-performance optical systems of practical importance.